Cryptococcosis ranks fourth in life-threatening infections in patients with acquired immune deficiency syndrome (AIDS, and the disease is a frequent and serious problem in individuals with compromised cell- mediated immune responses. Although it is well established that CD4 lymphocyte-dependent immune responses are essential for protection against Cryptococcus neoformans, natural effecotr cells make significant contributions to effective first-line host defense and, via activation by cytokines produced by immune T cells, may also function as effectors of acquired immunity. We have previously demonstrated that murine NK cells directly interact in vitro with C. neoformans and damage the organism. In addition, we and others have shown that NK cells participate in clearance of ryptococci from infected tissues. We now propose to extend our investigations of murine NK cells to determine if these cells also inhibit C. neoformans indirectly by secreting cytokines which potentiate the anti-cryptococcal activity of phagocytic natural effector cells. Culture supernatants from NK cells incubated with C. neoformans will be assayed for tumor necrosis factor, gamma interferon and colony stimulating factors by functional bioassays and ELISA, and induction of cytokines in NK cells will be confirmed by demonstrating cytokine specific messenger RNA using in situ hybridization. Cytokine containing supernatants from C. neoformans-stimulated NK cells and, for comparison, recombinant cytokines will be assessed for their abilities to activate polymorphonuclear leukocytes and macrophages to more effectively kill cryptoccocci. Although the functional activity of murine and human NK cells are similar, these two populations of cells recognize different tumor cell targets. Studies are proposed, therefore, to examine the anticryptococcal activity of NK cells from healthy human donors. In our preliminary studies, human peripheral blood NK cells, as well as T cells, attached to and damaged cryptococcal cells. We propose to define the ultrastructural and phenotypic characteristics of the human peripheral blood lymphocytes which interact with C. neoformans and compare and contrast the two different populations of human effector cells with regard to the requirements for cryptococcal cell interactions. In addition, we will determine if lymphokine-activated NK cells display greater direct anticryptococcal activity than do freshly isolated NK cells and if human NK cells have the capability to participate indirectly via cytokine production to enhance the anticryptococcal activity of other natural effector cells. Considering the potential role of NK cells in host defense in cryptococcosis, it is important to dissect the mechanisms by which mouse and human NK cells restrict C. neoformans growth and to delineate the immunomodulatory cytokines which may be involved in NK cell-mediated anticryptococcal resistance.